In the art of record sheet material involving transfer and receiving sheets, the use of basic, colorless, organic, chromogenic dye-precursor materials and acidic color-developing reactants has long been known. Commonly the transfer sheet includes a pressure-rupturable coating of isolated droplets of a solution of the basic dye-precursor for use against a receiving sheet which bears a coating of solid acidic color-developing material which may or may not be soluble in the solvent of the basic dye-precursor solution. Typical constructions involve transfer sheets bearing a coating of microcapsules having pressure-releasable liquid cores of a basic dye-precursor, such as Crystal Violet Lactone, dissolved in an organic solvent such as polychlorinated biphenyl. The receiving sheet for use therewith is typically coated with an organic solvent-soluble acidic phenolic resin or an organic solvent-insoluble clay such as attapulgite.
Microencapsulation is an effective method of retaining the liquid droplets of dye-precursor solution, but it is an expensive procedure. Nonetheless it is the dominant commercial method.
Hot melt transfer coatings of basic chromogenic liquid dye-precursor solution droplets dispersed in solid resins have the advantage of economy, but have not met with commercial success because less effective droplet retention is achieved thereby. These hot melt transfer coatings of the art function very well when first made, producing excellent images on mated acidic receiving sheets. However, with the passage of time (of the order of days or weeks), two serious problems develop due to the leakage or migration of the liquid content of the coating: (a) discoloration of adjacent acidic surfaces, either on the acid-coated opposite face of the transfer sheet or on the acid-coated face of a mated receiving sheet, and (b) hardening of the waxy resin hot melt matrix so that transfer by applied marking pressure becomes difficult and inefficient. Much the more serious of these problems is the first, discoloration due to leakage during storage.